I only just got into "Avatar" last month, but I still think this is a bad matchup. I've heard about this Gaara character (understand I am Naruto-challenged) and my understanding is that he has some form of geomancy. Well, considering at the current point in the show that Aang can willingly enter the "Avatar State" (it's a hyper-sort of powerup; think a watered down version of the Super Saiyan) I don't think there'd be much chance here.
A better matchup would be this guy against the Avatar character with a mastery for earth manipulation, Toph. Although, I've gathered from this guy's name that he can manipulate sand as well, and if sand came into play it would be bad for Toph; she can barely manipulate sand due to her methods of sensing.
Oh, and as far as making the series look like kid stuff? The plot is actually pretty clever, and the humor is actually funny. But things definitely get dark. In the season 1 finale, two characters snuff it. One in sacrifice, one mercillessly killed by Aang (though it's arguable that he wasn't consciously doing so). But if you want to see him pissed off, you should watch the episode following after his Bison is stolen. He lashes out at people out of spite, threatens (with some hefty malice in his voice and face) the original thieves and continually destroys their transports, and at one point becomes so pissed off that he slams his staff into a sand dune, resulting in the entire dune being upheaved and producing an explosion that resembles a mushroom cloud that's seen for miles.
I've also been surprised at how the show has a gift for making you feel a glancing blow of the trouble the characters get in while in domestic situations. Those who have watched the show will probably know what I'm talking about.